[PA Engineer]

For places/areas/pool owners who don’t get water professionally tested, high phosphate levels (ie, no chemistry used to remove from suspension) it would be called “high chlorine demand.”

I'm glad you posted this for selfish reasons too. Had to drain our swim spa. We have a greenhouse and the lower level is for the spa. Roofers dropped 3 bundles of shingles onto the glass shattering one of the roof panels. Had to drain it and spent untold hours removing safety glass (from everywhere). That was a year ago and I was planning on refilling this fall. That would have been a shock to find out. No pun intended.

What are they thinking? Phosphates cannot be removed in the sewage treatment cycle. It is going to be pumped directly into the waterways. I think you have identified the culprit in the blooms.

[PennsylvaniaMom]

Also, for some reason water mold (both white and pink) have been in an uptick as well in our non-chlorine pools. Phosphorous is NOT supposed to mess up biguanide (peroxide oxidizer) pools.

Seriously, “something” is up.